Liquid chromatography is very widely used for producing biopharmaceuticals, particularly for purifying a product molecule following its expression by fermentation, in cell culture, via transgenic plants or animals, or other means. A range of different modes of chromatographic separation are used, including ion exchange, hydrophobic interaction, size exclusion, affinity, metal chelate, reversed-phase chromatography, etc., often with more than one mode used in a sequence of stages in a given process. In each stage, a cycle of steps is run on a column filled with a particular chromatographic packing. These steps include loading of the feed solution, followed by a sequence of various solutions of different pH, ionic strength and/or concentrations of solvents, chaotropic agents, detergents, or specific eluent or cleaning agents. These steps are typically run in a semi-batch manner, usually including one or more feed, wash, elute, clean, and equilibrate steps. The cycle of steps is usually repeated multiple times with the same column and packing until the entire batch of product has been processed. At the completion of a batch the entire system is carefully cleaned and the column is either stored for reuse in another batch or the packing is discarded.
The use of chromatography for biopharmaceutical purification and production has a number of important challenges. One is that the separation system must be very extensively cleaned and the cleaning protocol must be capable of being validated. Cleaning before use is necessary to remove any residual contaminants from the assembly of the system and the packing of the column. During the batch, cleaning is needed to insure that the column and system are in an equivalent state at the beginning of each cycle. Cleaning before storage between batches is particularly critical to preserve batch-to-batch equivalence and to prevent the growth of microorganisms that could destroy the packing and contaminate later batches. If the equipment is to be reused for a different product, it must be completely cleaned of all traces of prior product to prevent cross-contamination. Sometimes the costs of cleaning procedures, the materials involved (such as very expensive high purity water) and the associated testing and documentation required for validation can exceed the costs associated with the process itself. This is particularly problematic for early-stage clinical production, where only a few relatively small batches may be made.
In recent years there has been a growing demand not only for larger annual amounts of some biopharmaceutical products but also for available production facilities to make an ever-greater number of products. As a result of these pressures a great deal of effort has been put into increasing the expression levels of the various recombinant production systems used to manufacture biopharmaceutical proteins, peptides and other products. The outcome has been a continuing and dramatic improvement in the productivity of bioreactors and other production facilities. The increased output has been a major benefit, both for the industry and for the consumers.
However, the increased quantities of product to be processed in each batch have created a major challenge for the chromatography systems used for downstream purification. With conventional technology, the systems must be made larger to handle the increased amount of product in the same time span. However, if the total annual production of product has not increased, the number of cycles over which the expensive chromatography packings are used is decreased, thereby driving up production costs significantly. That is, the same amount of total product is made in far few batches. As a result, the capital investment made in the chromatography equipment is not utilized efficiently because the equipment itself is not utilized to its full capacity. In addition, in some cases the size of the column is already at or near the practical limits, especially for systems that must be designed for validated cleaning. Simply enlarging the present chromatography systems is not a practical option.
One increasing trend in the biopharmaceutical industry is to use disposable process systems and components. Disposable systems eliminate the need for much of the cleaning process and the validation and testing associated with it because the systems can be produced pre-cleaned and pre-sterilized and are then discarded once the batch is completed. Plastic bags and tubing sets are now widely used in a variety of applications for solution and media preparation and storage, and increasingly sophisticated disposable units are being developed for use as bioreactors, mixers, fill/finish systems, etc.
The adoption of disposable systems for chromatography has been hampered by a number of factors. One is that the columns themselves must provide a number of key functions, including packing, flow distribution across the bed, pressure containment and aseptic operation. Although small, pre-packed columns, such as those widely used in laboratory applications, might be made inexpensively enough to be disposable, this approach would not be cost-effective for the large-volume columns (sometimes several hundred liters) used for production-scale purification. In addition to the columns, however, the rest of the chromatography system (including the significant number of valves, pumps and detectors) must also be made fully disposable (at least for the parts contacted by the process stream) in order for the benefits of disposability to be realized fully.
In the chemical industry, the challenge of very large scale chromatographic separation has been met through the use of multiple-column systems (including so-called simulated moving bed or SMB systems), in which a number of identical columns are connected together through a set of valves and pumps. Often these systems are configured so that some of the columns are engaged in loading the feedstream, while others are engaged in other parts of the process. At certain points in the process the valve positions and the consequent steps in which each of the columns are engaged are changed. These systems can be set up to run in a semi-continuous cycle, with all steps of the separation cycle taking part simultaneously on different columns in the system.
In addition to the use of smaller, more practical columns and continuous operation, multiple column systems have additional benefits. The effective capacity of the chromatographic packing can be higher because the elution process is only performed on a column that is completely loaded with product. In addition, proper design of the process can result in significantly reduced consumption of reagents per unit of product produced.
Multiple column systems have not been used significantly in biopharmaceutical production for several reasons. One reason is that some of valves used in these systems make it difficult or impossible to maintain the internal aseptic conditions required for biopharmaceutical production. More importantly, the large number of valves, pumps and other components presents a major challenge for cleaning and validation, both of which are critical and legally required steps in biopharmaceutical production.